Problem with the Hughes & Kettner Bassforce XXL - Grill Rattle

I looked around for anybody who faced anything similar and I had no luck, which brought me here: I have this amp, and it sounds great besides my recent discovery that the metal grill on the front of the amp tends to shake a ton, either at higher volumes or in the neighborhood of 61 Hz.

Has anybody owned this amp, or have people resolved similar problems on other amps/cabs? What should I do?